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Impulse Reaction TurbineImpulse Reaction Turbine

Gas Turbine, Systems II

In WW-II and shortly thereafter, piston powered aircraft peaked in power, performance and complexity wise. Power went up to over 4000 bhp for large multi-row radial engines. Only to be defeated by the jet, which was developed by (among others) Germany's Dr. Hans von Ohain and separately in the UK by Sir Frank Whittle. Its principles are based on the "Aeolipile" of the ancient Greek scientist Hero and other great thinkers like Leonardo da Vinci and the laws of Isaac Newton.

Compared to a piston, the gas turbine has less parts and the moving parts rotate in only one direction without stopping and accelerating as the pistons normally do in an engine. Thus, a running gas turbine is basically free of the vibrations normally found in piston models, which translates in much longer service life (TBO) and higher reliability.

After the air is compressed and fuel is injected the combustion process will add heat to the process. This energy is used to drive the turbine and the remaining thrust pushes the aircraft forwards.

Engine Combustion & Exhaust

Gas Turbine Combustor

Usually made of several cans or combustors. It burns the fuel under difficult conditions as changing RPM, altitude, air pressure, temperature and various TAS. There are three main types: can, annular and can-annular. Of all the airflow entering the can only about 20 % is used to burn the fuel, another 20 % enters the can through holes in the side of the can and the rest is used for cooling after combustion is completed.

Air temperature from the compressor runs between 200 and 400 °C and goes further up to 600 to 1650 °C in the cans. Flame temperatures are some 2200 °C.

Can combustorsCan Combustor
Can combustors
are used with radial flow compressors as they can be positioned around the compressor turbine axis and the air flows from the diffuser direct into the can. Annular combustorsAnnular Combustor
Annular combustors
are suited for the axial flow compressor. Fuel enters the cans upstream and air is used to center the flame without it touching the can liner. Secondary air is used to cool the cans.

General Electric Aviation uses a double annular combustor (DAC)Double Annular Combustor
Double Annular Combustor
for reduced CO2 and nitro oxide (NOX) emissions. Now lets wait and see who will build the first triple annular combustor.

Can-annular combustorsCan-Annular Combustor
Can-annular combustors
are a combination of individually placed side by side cans to form a circle inside an annular chamber. Thus combining the best features of both types in one. Providing an even temperature distribution along the HP turbine without hot spots should a fuel nozzle become blocked.


Gas Turbine EngineGas Turbine Engine

Drives the compressor and the engine accessories and thereby by using a lot of power, sometimes upto 75%. The hot exhaust gases require careful turbine blade manufacturing and special alloys offering high resistance at these temperatures. Cooling air is used flowing radially across the turbine disk preventing excessive heat reaching the turbine blades.

Impulse & reaction turbine

The blades rotate partly by impulse and reaction. Impulse motion is where the rotating turbine blades catches the incoming high velocity gas on a special formed blade and turning them in opposite direction at a lower velocity. The turbine rotates then due to the kinetic energy imparted on the blades. Reaction is where the rotating blades start moving when the hot gases hit the bottom of the blades creating a reactive force as the cambered wing of an airplane.

Rotating the shaft

For both the same principles apply: gas velocity is build up in the stationary nozzles then decreased in the rotating blades where energy from the hot gases is converted to kinetic energy and then into mechanical energy by the rotating shaft.

It is clear that all these energy conversions can not be done at 100 % efficiency, there are heat and friction losses in every mechanical process. As a result of these losses thermodynamic and mechanical efficiency in a turbine is around 90 to 92%.


Increasing the thrust of a GTE is done by discharging the exhaust gases at a higher velocity than the turbine gas velocity. Exhaust nozzles collect and straighten the gas stream. When the gas exits the turbine the flow speed is reduced by diffusing in conjunction with a tailcone lowering tailpipe friction. Before the gas exists, its velocity is increased by the converging tailcone.

EGT probes

EGT temperatures are also measured here at the exhaust by a number of parallel wired EGT probes and then taking the reading. It is of the utmost importance that upstream temperatures are not exceeded and remain within limits for the engine. This instrument is also important when starting a gas turbine engine, discussed on our next page.

Written by EAI.

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